If you are new to pottery and have just fired your first piece or pieces of pottery, you may have noticed that they are slightly smaller when they leave the kiln than when they enter it. Why does clay shrink when fired? It is important to have a deep understanding of the physical and chemical processes that soil undergoes during kiln drying and firing. This understanding will help you prevent clay and fire defects in your project and help solve many of the problems you will encounter with your pieces such as cracks, splits, cracks, and glazing defects.

Losing and gaining mass for clay

Clay shrinks during firing for many reasons. First it loses water. Second, it undergoes chemical and physical changes that make it denser than in its original state. The closer the clay particles are packed, the smaller the ceramic. Clay shrinks during drying and firing. Different clay bodies shrink at different rates, which can be as much as 4%, or as much as 15% for some clay bodies. In general, clay consists of 20% water and 9% organic matter. The result of this is that the blasted piece can weigh almost 30% less than its original weight. 

Although the clay comes into the kiln dry (to the bone), it still contains water. This water is residual moisture and also water that is chemically bound to the clay particles. When the mud is burned, the remaining moisture evaporates and the chemically bound water is forced out of the mud. The clay shrinks with this drying. Even a percentage point can make a difference in the final product depending on your need for accuracy. Therefore, your design must take into account the reduced temperature you are burning. 

Why is it important to let the clay dry slowly?

Note that the larger the piece, the more shrinking movement will occur. It is useful to know that the larger the piece, the more important it is to dry it evenly (slow drying will ensure even drying). If one side dries faster than the other, the drier side shrinks the wetter side and creates stress in the clay, so cracks can occur. Air drying Wet clay contains a lot of water, at least 25% water. When the clay starts to dry, the water evaporates from it. When this happens, the clay particles become closer together, resulting in shrinkage. Many problems with clay are caused by uneven drying rates that create stresses in the clay. Sometimes this stress manifests itself immediately as cracks or deformations, other times during or even after a fire. Therefore, it is important to ensure that the drying is fair. This is achieved by ensuring the same thickness throughout the piece, drying slowly and even delaying the drying of some parts. 

Clays with very fine particles shrink more than clays with larger particles

Porcelain clay has very fine particles that make it very plastic and also very shrinkable. These bodies have the greatest strength in the dry state. Clay as sculptural bodies are small. (Grog is clay that has been fired and then ground into various particle sizes.) These bodies do not shrink much because they initially have a low water content and also provide channels for moisture to reach the surface. These are called "open bodies". When the water between the clay particles has evaporated and all the remaining soil particles are in contact, dry shrinkage is complete. This is called the skin-difficult phase. The particles themselves are still wet, but drying does not cause further shrinkage. 

Clay cools after firing 

Another point where the clay shrinks in the firing process is as the pottery begins to cool. There are things in clay that shrink when it cools. One of these substances is called Cristobalite. Cristobalite is a crystalline form of silica. Silica is one of the components of clay that forms glass in clay at high temperatures. When Cristobalite cools, it dries suddenly. The contraction was about 0.8% by volume. This reduction occurs at window 86F (30C) centered at window 392F (200C). Due to sudden shrinkage, it can cause cooling cracks in the clay. By the time the ceramic cools, it is no longer plastic. However, it is hard and brittle, and sudden contractions can cause the ceramic to crack. This is also why it is important to slowly cool the ceramics after firing. 

Conclusion

To answer the question, does clay shrink when fired? The answer is yes. In fact, it shrinks at every stage of the process, from the moment it is taken out of the plastic bag and the potter starts working on it. In general, the biggest declines are due to atmospheric drying, where the clay changes from hard to fur dry. The second dramatic change in volume is when the glaze is fired. However, different soil bodies react differently to heat and the exact percentage and time of reduction depends on the type of soil used.